Stand to leave part of sports ground quietly - it’s not upright (5,5)
I believe the answer is:
grand piano
'upright' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for grand piano that I've seen before include "one getting tickled?" , "responsible for producing notes ..." , "Large stringed instrument" , "One's three-legged" , "Large instrument with horizontal strings" .)